Secret/credential file handling: decode in /tmp/opencode only, never print, delete after use

When Burak Yigit Kaya hands over a secret (e.g. Apple API key bundle) via a file, he expects: decode/process it only under /tmp/opencode/ (never write into the repo), never read or print its contents into chat output, and delete the file once done. He prefers dropping secrets into a file for the assistant to process directly over pasting them into chat. This extends the general WIP-artifact rule [[019f7135-8af9-78ae-833c-8decd37d78b1]] specifically to sensitive credentials, where the added constraint is never surfacing the raw value in output.